The NCI is carrying out a major evaluation of selected screening procedures for the early detection of prostate, lung, colon, and ovarian cancer (The PLCO Trial). The Trial will include 74,000 screening arm participants (37,000 men, 37,000 women; age 55-74) and an equal number of non-screened controls. Questionnaire data and biologic samples are collected in this population for use in studies on the etiology of cancer and related diseases and for the evaluation of early markers of cancer development. Within the Trial, etiologic studies for specific cancers and related conditions will be phased in as sufficient numbers of cases accrue. Currently, we are evaluating genetic and environmental (including lifestyle-related) determinants of risk for adenomatous polyps, to identify causes of the early changes in the pathologic sequence leading to colorectal cancer.